It is very funny that, our own pets may sleep on the 2 wheeler seat, but they will never scratch the seat.
But stray cats after sleeping, when they wake up stretch and scratch! they may also scratch the tyres of your car also.
But as you said it is cruel to scare them away.
i used to keep an old tarpaulin on my seat to keep stray cats away. Bu the best remedy was my own cat (now i don't have any) he used to scare away others, so that he can relax on the seat!
i would like to share a story,
some years back, i was staying at a lodge near my workplace. it had a covered parking space for 2-wheelers.
A stray dog gave birth to 3 puppies in a corner of the parking lot. Since it was very weak and some cruel fellow had beaten it and broke her leg, it was a pity sight.
We used to feed her, at first she was scared of us, we normally left the food near her hiding place and left.
Later she became adapted to us.
She used to scare away cats from our (3 of us) bikes! we used to park near her sleeping place to give her additional protection.
At times she used to bark at night, and in the morning we found that petrol were missing from other bikes.
but none stole from ours.
One day when we reached the lodge we found that the Municipality was there and killed the 3 puppies and took their mother with them.
it must have those petrol thieves, because after that we began losing petrol everyday, and we fitted the lock type petrol taps.
then after 2 weeks the dog was back, searching for her puppies, it was a sorry sight, her body was full of bruises. after that she began a careful watch at night. and we too made arrangements with others on the lodge to catch the petrol thieve. we were able to scare one away at night by jumping at him when the dog barked, but he managed to slip away.
Later that week we found the dog dead, he must've poisoned it. As for the petrol thieves we filed a complaint at Police station and they used to visit there during beat, and after a short while we all moved out.